Field of the Invention
The invention relates to a high-temperature-resistant component and a method of protecting a high-temperature-resistant component against oxidation.
U.S. Pat. No. 4,321,310 describes a composite coating system for protecting a metallic component, in particular at high temperatures. In particular, a coating system for a gas turbine blade is described. A gas turbine blade is subjected to great stresses by high temperatures and strong oxidation. In order to be able to withstand the high temperatures, gas turbine blades are frequently made of a nickel-based or cobalt-based superalloy which has a sufficiently high melting point. In general, it is also necessary to protect the surface of such a gas turbine blade against oxidation. This can be achieved by means of an MCrAlY coating. Here, M is at least one of the elements iron, cobalt or nickel, Cr is chromium, Al is aluminum and Y is yttrium. Oxidation protection is achieved by a very oxidation-resistant aluminum oxide being formed on the surface of such a coating. In the document cited, such a coating on a gas turbine blade is additionally provided with a ceramic coating. This ceramic coating acts as a thermal barrier layer. Between the thermal barrier layer and the oxidation layer, there is arranged an aluminum layer. This improves, in particular, the adhesion of the ceramic layer to the antioxidation layer.
WO 97/07252 describes a product for conducting a hot, oxidizing gas. In particular, this is a highly thermally stressed component of a gas turbine.
It is stated that the oxidation resistance of metallic protective layers comprising MCrAlY is impaired over the course of time by the protective layer being continuously depleted in aluminum. Such depletion occurs as a result of the oxide film being continually worn away during use of the component and aluminum continuously diffusing from the protective layer to the surface. Accordingly, the maximum life of a protective layer depends on its aluminum content. A high proportion by weight of aluminum in an alloy for a protective layer is therefore desirable in the interests of longevity. However, a high proportion by weight of aluminum leads to embrittlement of the alloy. This is because the aluminum is not present in elemental form in the alloy, but at least a significant proportion of it is in the form of intermetallic compounds, in particular intermetallic compounds of nickel and alumium or cobalt and aluminium. Different properties, in particular different coefficients of thermal expansion, of these intermetallic compounds result in the embrittlement mentioned. This greatly restricts the proportion by weight of aluminium in the protective layer. It is stated that alloying of gallium into the protective layer ensures the desired oxidation resistance, with the alloying-in of gallium simultaneously leading to an improvement in the ductility of the protective layer, i.e. the brittleness of the protective layer is reduced.